Field of the Invention
The present invention relates to a power saving control technique in a communication apparatus.
Description of the Related Art
It is well known that in Ethernet® the energy use efficiency is higher as the link speed is higher during data transfer, and the power consumption can be lower as the link speed is lower during an idle state in which no data transfer is performed.
Based on this characteristic, a technique called LPI (Low Power Idle) has been standardized as the EEE (Energy Efficient Ethernet) standard in IEEE802.3az in order to save power in Ethernet. In LPI, the state transits from a normal state to an idle state while there is no frame to be transferred by Ethernet. In the idle state, by powering off unused circuits of a PHY and MAC, it is possible to reduce the power consumption in Ethernet without decreasing the link speed. Since LPI is an optional function of Ethernet, the self apparatus notifies, in auto negotiation upon establishment of a link, a partner apparatus that it is an LPI-capable apparatus. Therefore, as a result of auto negotiation between the self apparatus and the partner apparatus, whether to use LPI is determined in addition to a link speed and duplex.
On the other hand, in order to save power of a communication apparatus for performing communication using Ethernet, there has been conventionally proposed a method of selecting a low link speed in a standby state in which the communication apparatus waits for reception with low power consumption. For example, Japanese Patent Laid-Open No. 2009-239870 discloses a method in which the communication apparatus side determines the communication mode of a network interface apparatus in a state in which the communication apparatus has transited to a power saving mode, and causes the network interface apparatus to set its mode to the determined communication mode. That is, in Japanese Patent Laid-Open No. 2009-239870, power saving is achieved by setting the network interface apparatus so that the link speed in the power saving mode becomes low. Furthermore, Japanese Patent Laid-Open No. 2004-243533 discloses an image forming apparatus for, in a network communication standby state, selecting a mode in which the operating frequency of a PHY is low, and supplying a clock signal. Setting a low operating frequency for a PHY of Ethernet amounts to setting a low link speed.
In a state in which an apparatus for performing network communication by connecting to Ethernet is inactive and is waiting for reception from a network, the apparatus preferably reduces the power consumption as much as possible. If a communication apparatus supports an LPI function and another communication apparatus as a link partner also supports LPI, it is possible to reduce the power consumption while no data transfer is performed. It is, therefore, possible to efficiently obtain the power saving effect in a standby state in which almost no communication is executed.
In auto negotiation, however, a link is established by selecting a maximum one of advertised link speeds at which intercommunication is possible. If, therefore, LPI is not enabled, it is impossible to reduce the power consumption of a network interface. As described above, in the standby state of the communication apparatus, it is possible to save power by selecting a low link speed to set a link mode. On the other hand, however, when the state transits from the normal state to the standby state and when the state transits from the standby state to the normal state, it is necessary to re-establish a communication link. A temporal overhead therefore unwantedly occurs from when a link is established by renegotiation until the apparatus returns to a communicable state after entering a link-up state. Therefore, this is inconvenient if it is desired that the communication apparatus quickly returns from the standby state to the normal state.